This invention relates to a weak magnetic field measuring system having dc-biased SQUIDs (Superconducting Quantum Interference Devices), and particularly to a system for biomagnetic diagnosis.
The SQUID (Superconducting Quantum Interference Device) is a high-sensitivity magnetometer making use of the superconducting Josephson effect. Particularly, the dc-SQUID is widely used because of its low noise. Its structure and operation are disclosed in, for example, Journal of Low Temperature Physics, Vol. 25, Nos. 1/2, pp 99, 1976.
On the other hand, a system has been published for simultaneously measuring a magnetic field at a plurality of measurement points by providing many SQUID magnetometers, as in, for example, "31-Channel DC SQUID Gradiometer Array for Biomagnetic Diagnosis" (Abstracts of 1989 International Superconductivity Electronics Conference SS-1 (1989)).
In addition, a method for signal source estimation by use of measured data from multi-channel magnetic field measuring system is presented in "Measurement of Somatosensory Evoked Magnetic Fields Induced by Electrical Stimulation", the Journal Magnetics Society of Japan, Vol. 13, No. 3, 1989, pp 508 .about.512. According to this paper, the positions, strengths and directions of the current dipoles assumed within the head are determined to provide the minimum sum of square differences between the theoretical magnetic field distribution established on the surface of the head by the current dipoles and the experimental magnetic field distribution.